During well logging operations, logging tools are lowered through a well bore into an earth formation. Logging tools are suspended in the well bore by cables. These tools detect and measure phenomena that are reflective of formation characteristics while the tool is raised and lowered in the well bore.
Unfortunately, during these logging operations, it is not unusual for a logging tool or even the cable to become stuck in the well bore. Various factors, such as well bore deviation, contribute to tools and cables becoming stuck or lodged in the well bore. When it becomes necessary to remove various objects (fish), such as a segment of drill pipe or a subsurface tool from a well bore, a procedure known as "fishing" is implemented. During a wireline fishing operation, a device known as an overshot ( a device that engages the top of a logging tool by use of a grapple and retains the tool) is lowered down the well bore to the location of the stuck tool. Once the overshot engages the stuck tool and dislodges the tool or frees the cable, surface retrieval equipment pulls the overshot and the logging tool to the surface where the tool is retrieved. The following description of the fishing procedures assumes the reader has the basic knowledge of techniques and procedures involved in fishing for well logging equipment.
The typical equipment and steps involved in a wireline fishing operation are described in FIGS. 1A-1D. When fishing for a stuck tool, the engaging overshot 16 that will engage the tool is lowered into the well bore 13. In order to lower an engaging overshot into the well bore, the cable 12 supporting the stuck logging tool must be severed to thread the cable through drill pipe 17. Threading is the process of pulling the cable through the center of drill pipe as the engaging overshot is lowered into the well bore. As shown in FIG. 1A, a supporting T-Bar 11 is attached to the cable 12 to secure the cable and prevent it from being pulled into the well bore 13 by the weight of the portion of the cable suspended in the well bore. After attaching the T-Bar to support the cable, the cable is severed at the surface. However, by severing the cable, mechanical and electrical contact between surface logging equipment and the logging tool downhole ceases.
FIG. 1B shows the conventional reconnection procedure that is performed to re-establish the mechanical connection between the tool and surface logging equipment (reconnect the severed cable). This cable reconnection step incorporates a spear 14 attached to one end of the severed cable and another cable overshot 15 attached other end of the severed cable. The spear slides into the overshot socket to form a union and re-establish the mechanical connection of the cable. To insure reliability of this union, stress tests are performed on the union. (Note in a fishing procedure there is no re-establishment of the electrical communication between downhole and surface equipment).
The next step in the convention fishing procedure is to pass the engaging overshot 16 over the cable 12 and thread as shown in FIG. 1C. During the attachment of both the engaging overshot and first stand of drill pipe 17, the spear 14 and cable overshot 15 are disconnected to allow for passing through of the cable through each piece of equipment. Field personnel who perform fishing operations are aware that the cable is attached to the tool in the well bore at one end and to surface equipment at the other cable end. Therefore the easiest way to install the fishing equipment is to sever the cable. Consequently, during this process each time a piece of equipment or drill pipe is installed, the cable is disconnected, threaded through the drill pipe, and reconnected, using the spear 14 and overshot 15 (these devices are also known as latches).
Each time a stand of pipe is attached, the engaging overshot is raised to the top of the drill pipe stands and then lowered through each additional stand of pipe. The depth of the stuck tool determines how many stands of drill pipe are connected to the engaging overshot to enable the overshot to engage the stuck equipment.
FIG. 1D shows the fishing equipment installed in the well bore. In the figure, a logging tool 20 is shown in an open well bore. The casing 19 inside the well bore may or may not extend to the depth of the tool. The engaging overshot 16 is lowered into the well bore by extensions of drill pipe 17. The spear 14 and cable overshot 15 shown as 18 provide a mechanical connection and support for the cable. A spear and C-plate (not shown) provide support for the cable during the attachment of additional stands of pipe, which require the disconnecting of the spear and overshot.
In FIG. 1D, the engaging overshot 16 contacts the stuck tool 20 and dislodges the tool or frees the wireline (cable). Once the overshot engages the tool 20, the entire assembly (logging tool, engaging sub and drill pipe) can be raised to the surface. Although, the present methods are successful in retrieving stuck equipment, there are many concerns associated with the present method. One major concern is the time required to retrieve a piece of equipment from the well bore. It is not unusual to take up to 39 hours or about a day of rig time to retrieve a stuck tool and another day to prepare the well for additional logging.
During current tool fishing procedures, the support cable is severed at the surface to enable work crews to pass the fishing equipment over the cable. This equipment will be lowered into the well bore via a drill pipe 17. Fishing equipment is attached to both ends of the severed cable. Work crews then re-attach the severed cable. However, the re-attachment is only for mechanical purpose. In current fishing procedures, there is no re-establishment of the electrical communication in the cable. Because there is no re-establishment of the electrical connections, no further logging can occur while retrieving the tool from the well bore. In addition, work crews do not have the option of continuing the logging run if it is desired once the tool has been dislodged from the well bore. This situation increases the length of time required to do a job and increases the overall cost of the job. The cost is increased in several ways: 1) increased length of time the logging crew is occupied with this job; 2) rig and support time continue to mount (not uncommon to be as much as $4000/hour); 3) cost of doing the fishing procedure; and 4) in a production well increased logging time means loss of valuable production time.
Therefore a method and system of retrieving equipment stuck in a well bore are needed that can provide more efficient use of the retrieval time and the fishing equipment.